Eleven Oil Producing communities and two non-oil producing communities were identified within Delta state and selected for sampling. Figure 1 shows the map of sampling communities within the state. Sediment samples

were collected at approximately 1.2 km (1) and 800 m (2) away from visible oil exploration activities within these selected communities. The collected samples were distributed to obtain an average of the concentration of radioactivity for a uniform distribution over the area of interest with samples from the non-oil producing communities as control. The sample collection equipment, containers and sample preparation areas were be kept clean to avoid contamination. Disposable containers (plastic bags, aluminum trays, etc.) were used whenever possible. The box corer; a marine geological sampling tool for soft sediment and suitable for any water depth with minimum disturbance of the sediment surface by bow wave which is important for quantitative investigations, was dropped into the bottom of the river for sediment samples collection.

All samples were collected and prepared according to equipment’s specification. Hyper Purity Germanium (HPGe) Detector (Canberra, model gc. 8023) at the National Institute of Radiation Protection and Research, University of Ibadan, Nigeria was used for counting and detection of radionuclide content of sediments samples from all selected locations. Collected samples were transported to the research laboratory on collection days. All samples were allowed to dry at room temperature and pulverised at the laboratory, sieved and put inside two sets of labeled transparent plastic containers. Sediment samples containers were carefully sealed for 4 weeks in order to establish secular radioactive equilibrium between the natural radionuclides and their respective progenies.

Hyper Purity Germanium (HPGe) Detector was used for counting and detection of radionuclide content of all samples. The instrument have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV. Cooled to liquid nitrogen temperatures (196˚C), it produces spectroscopic data and pulses proportional to the captured photon energy. The HPGe detector used is a P-type and has a diameter of 78 mm, length 69.6 mm, 16 K channels with relative efficiency of 80% and a resolution of 2.3 Kev at the ⁶⁰Co line of 1.33 Mev. The spectrometer was equipped with the necessary electronics. A Digital Spectrum Analyzer (DSA-1000) was connected to the HPGe, providing the detector with the required voltage and pre amplifier model 2002csl was employed for the pre amplifying stage. Counting time was 28,800 seconds. Multi-gamma ray standard source (mgsAm241) was used for the equipment calibration. Standard point sources gamma emitters (Caesium, Cobalt and Americium) were used for the energy calibration to determine the radionuclide present in the samples within a wide range of photo peaks and efficiency calibration was done using volume source of the same geometry as the samples in determining the activity concentration of the radionuclide present in the samples.

The efficiency of specific spectrometer is given by the following equation

η = Efficiency of specific geometry, Nc = Net count, A = Activity of standard source.

= Gamma emission probability or gamma yield and T = Time of Counting (seconds).

An efficiency calibration curve is produced using Excel worksheet, polynomial fittings are performed with a degree that gives the best R2-value that fit very well the two well-known parts of the calibration curves. Two fitting curves were performed separately to reach the maximum agreement with the measured data. With the equations produced from the fitting curves, activity concentration of U²³⁸, Th²³² and K⁴⁰ (Bq∙kg⁻¹) were calculated based on the measured efficiency of the detector and the net count rate as follows:

M = Mass of sample (kg).

Uncertainties in the activity is calculated as given by [13] in Equation (3)

where ΔA, ΔNc, Δη, and ΔM are uncertainties in the activity, the count rate, the efficiency, the gamma yield and the mass, respectively.

The activities of natural radionuclides K⁴⁰, Th²³² and U²³⁸ in sediment have been analysed in the oil producing communities of Delta states. Table 1 shows the Coordinates of sampling locations, samples ID and activity con- centration of the main gamma-emitting natural radionuclides. The Activity values of K⁴⁰ ranged from 32.47 Bq∙kg⁻¹ to 525.1 Bq∙kg⁻¹ with an average concentration of 302.15 Bq∙kg⁻¹. The values of Th²³² ranged from 6.31 Bq∙kg⁻¹ to 19.33 Bq∙kg⁻¹ with an average concentration of 11.66 Bq∙kg⁻¹. The values of U²³⁸ ranged from 4.45 Bq∙kg⁻¹ to 18.69 Bq∙kg⁻¹ with an average of 8.66 Bq∙kg⁻¹. Activity concentrations at the control site ranged from 3.15 to 4.80, 2.27 to 4.18 and 1.15 to 1.76 for K⁴⁰, Th²³² and U²³⁸ respectively. Figure 2 which is the radioactivity mapping of study area, shows the level of activity concentration at each sampling location on the map of Delta state. The worldwide concentration of the radionuclide K⁴⁰, Th²³² and U²³⁸ have averages in sediment samples of 40 Bq∙kg⁻¹, 40 Bq∙kg⁻¹ and 370 Bq∙kg⁻¹ respectively [19] . Table 2 shows concentrations of the natural radionuclides in sediments samples from different parts of the world compared with those of the present study. The present results demonstrate that the average activity concentrations of these radionuclide are less than the worldwide average concentrations.

Absorbed Dose is calculated by introducing the dose conversion factor for each radionuclide being considered.

where C_K, C_Th and C_U are activity concentration of K⁴⁰, Th²²³ and U²³⁸ respectively. Table 3 shows range of activity concentrations, mean concentration, Absorbed Dose and Annual Effective dose equivalent of K⁴⁰, Th²³² and U²³⁸ in the sediment samples. Figure 3 shows the variation in the total absorbed dose in sediment samples across sample locations in Delta state.

A common index used to compare the specific activities of materials containing U²³⁸, Th²³² and K⁴⁰ by a single quantity, also takes into account the radiation hazards associated with them [3] [6] . The radium equivalent is related to both the external γ-dose and the internal α-dose from radon and its progeny. The activity index provides a useful guideline in regulating the safety standards. The radium equivalent activity index is calculated as given by [5] in Equation (5).

where; C_Ra, C_Th and C_K, are the radioactivity concentration in Bq∙kg⁻¹ of ²³⁸U, ²³²Th and ⁴⁰K respectively.

The above equation is based on the assumption that 370 Bq∙kg⁻¹ of ²²⁶Ra, 259 Bq∙kg⁻¹ of ²³²Th, and 4810 Bq∙kg⁻¹ of ⁴⁰K produce the same gamma-ray dose rate. The values of the Radium Equivalent doses are given in Table 4 and ranged from 13.48 Bq∙kg⁻¹ to 52.86 Bq∙kg⁻¹ at sampling location with known anthropogenic input (oil exploration) and ranged from 5.19 Bq∙kg⁻¹ to 5.70 Bq∙kg⁻¹ at the control sites. The permissible maximum value of the radium equivalent activity is 370 Bq∙kg⁻¹, which corresponds to an effective dose of 1 mSv∙yr⁻¹ for to the inhabitants of dwellings [19] .

The gonads, the bone marrow and the bone surface cells are considered as organs of interest by [19] . The AGDE for given activity concentration of U²³⁸, Th²³² and K⁴⁰ is calculated using the Equation (6)

where C_Ra, C_Th and C_K are the radioactivity concentration of U²³⁸ Th²³² and K⁴⁰.

Values of AGDE as shown in Table 4 ranged from 66.54 mSv∙yr⁻¹ to 207.5 mSv∙yr⁻¹ with an average of 137.02 in study areas with visible anthropogenic input and 20.8 to 25.39 with an average of 23.09 at the control sites. The standard UNSCEAR value for AGED is 300 mSv∙yr⁻¹.

The external hazard index, a relation that quantifies the exposure factor [8] is an evaluation of the hazard of the natural gamma radiation and given by [12] in Equation (7)

where C_Ra = C_U, C_Th and C_K are the activity concentrations of ²³⁸U, ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in Bq/kg. Table 4 show that the values of H_ex ranged from 0.023 to 0.23 in all study locations which is less than 1. Hence, the radiation hazard at study areas are negligible. Figure 4 shows the variation of some radiological indices

In addition to external hazard index, radon and its short-lived products are also hazardous to the respiratory organs. The internal exposure to radon and its daughter products is quantified by the internal hazard index H_in, as given by [20] in Equation (8)

where C_Ra = C_U, C_Th and C_K are the activity concentrations of ²³⁸U, ²²⁶Ra, ²³²Th and ⁴⁰K, respectively, in Bq/kg. Values of H_in ranged from 0.027 to 0.28 in all study locations. The values of H_in is less than unity hence the radiation hazard is negligible.

In estimating the annual effective dose equivalents, the conversion coefficient from the absorbed dose inairand the outdoor occupancy factor were considered. A value of 0.7 Sv/Gy was used for the conversion coefficient from the absorbed dose in air to an effective dose received by adults and 0.2 for the outdoor occupancy factor (UNSCEAR 2000). The annual effective dose equivalent was calculated from equation

The values of the calculations are given in Table 3. The annual effective dose rate values in the sites with known anthropogenic input as shown in Table 4 varied from13.48 μSv/y to 52.86 μSv/y while those from the control sites varied from 5.19 μSv/y to 5.70 μSv/y. The world average annual effective dose equivalent (AEDE) from outdoor UNSCEAR (2000) 70 μSv/y. Therefore, with the obtained value slower than the world average value they pose no risk to the environment.

The average AEDE value was calculated as 69.8 μSv/y in Istanbul (Turkey) [9] , 152 μSv/y in the Xiazhuang Granite Area China) [21] , 314.1 μSv/y in the Southeast part of Eskisehir (Turkey) [15] , and). These average values are generally higher than the present value.

Radioactivity mapping of the study area was carried out using the Surfer programming software. Surfer is a contouring and 3D surface mapping program that runs under Microsoft Windows. The program was used to converts data generated for activity concentration at different coordinates of location within the state into maps. Figure 2 shows the radioactivity mapping of study sites within Delta state for K⁴⁰, Th²³² and U²³⁸ in sampled sediments.